1. Field of the Invention
This invention relates to a system for tracking and surveillance radars which are susceptible to mainlobe jamming. In particular, the invention discloses a standoff jamming ECCM technique that removes a random noise source in the mainlobe of the radar beam. The noise source is anticipated to be random in phase and/or polarization.
2. Discussion of the Prior Art
Devices have been known in the past that improve the resistance of tracking and surveillance radars to mainlobe jamming. One such method employs a dual radar which measures the signal level and phase of two orthogonally polarized channels to determine the receive polarization of the noise source. A polarization control network including RF/IF hybrids and electronic phase shifters attached to a dual polarized antenna is employed to change the receive polarization to be orthogonal to the noise polarization. The significant drawback of this method is its inability to isolate and measure the noise vector alone. Thus, the objectionable possibility exists of having the desired target signal contribute to the estimate of the noise vector. An additional disadvantage of this method is that the target vector may have the same direction sense as the noise vector, which results in the removal of the desired signal. Yet another significant limitation of this method is that the time-constant of the polarization control network (PCN) is typically orders of magnitude greater than the sample rate of the radar. Thus, a polarization agile noise source, whose bandwidth is greater than the equivalent bandwidth of the PCN control system, will not be removed.
Another method of reducing the susceptibility to mainlobe jamming involves the direct application of side-lobe canceller technique to dual polarized radar. This approach typically uses open loop cancellers to adaptively form a weight to remove the correlated signals between two orthogonally polarized receive channels. Some of the advantages of this approach over the first method described above include first, the cancellation ratio is a function of the dynamic range in the signal processor used to form the weights, and second, the equivalent time width of the sample window used to average the weights is smaller than the typical time constant of the PCN discussed hereinabove in connection with the first prior art method. However, the sample window width used to calculate the correlated signal component is still many samples at the radar's instantaneous bandwidth. Thus, a polarization agile noise source whose instantaneous bandwidth approaches that of the victim radar would require a sample window width of unity. In such a case, complete cancellation of target and noise signals will occur.
A third method which has been considered depends upon the application of main-lobe notchers. The implementation of a system described by this third method is essentially similar to the well known side-lobe canceller method. The main-lobe notcher is used to cancel high duty cycle interference which enters the radar's main-lobe from a single direction in space. The drawback with this method is that the target signal must be at a different angle from the null in order to be detected.
A fourth method known to applicants for coping with the problems of main-lobe jamming relies on the modification of a technique sometimes called "main-lobe noise cancellation". FIG. 1 schematically illustrates a prior art system for noise cancellation in the main lobe in accordance with this technique. Referring to the drawing, reference numeral 13 denotes a fixed time delay, 18 mixers, 20 IF amplifiers, 22 a local oscillator, 24 variable time delays, and 26 and 28 cancellers. This approach is essentially a radar ECCM technique that degrades the effectiveness of main-lobe jamming by using two or more physically separated radar receiving sites. For example, where an attack aircraft 10 is protected by a nearby jammer, the radar transmits only with a primary antenna 12 and receives reflections on the primary antenna 12 and two spaced auxiliary antennas 14 and 16. By properly delaying the individual signals which enter the auxiliary antennas, these signals can be correlated and used to cancel the noise signal. In order for the target signal not also to be cancelled, this spinoff of the ECCM technique relies on the fact that the radar returns from a complex geometrically shaped moving target will be different and partially non-correlated at the locations of the two auxiliary antennas 14 and 16 as compared to the principal antenna 12 due to the bistatic angle 12-10-14 and 12-10-16 as shown in FIG. 1. When the noise and target signals are applied to the cancellers 26 and 28 only the partial non-correlated target signal component will pass through. While successful to some degree, this method suffers from the disadvantages that the cancellation ratio is dependent on the target scattering matrix to return non-correlated signals; it will fail to detect a target when the polarization agile noise source's instantaneous bandwidth approaches the radar's instantaneous bandwidth; and it results in the need to continually determine the relative time delays of the individual noise signals in order to achieve cancellation.
A conventional radar jammer is in the form of a platform (e.g. a plane or a warship) associated with target platforms (e.g. airplanes or missiles) which the jammer seeks to protect from detection by hostile radars. In response to a hostile radar, the jammer sends out a signal larger compared to the radar echo the target platform would return, and of sufficient duration to ensure that the jammer signal covers the target echo. In short, the jammer buries the target echo both in time and magnitude.
A countermeasure to such a jammer is to detect and estimate the jamming signal, and calculate signal weights to cancel out the jamming signal. However, this does not preserve information about the signal's polorization. Modern radars also use the polarization of radar echoes to gain additional knowledge of the target, such as geometry and composition. A jamming signal that periodically changes its polarization, or, for that matter, a fixed polarization noise jammer, will defeat any countermeasure that did not adapt to the changing jammer polarization. Accordingly, any countermeasure to such a jammer would be most welcome.